Protocol for memory power-mode control

ABSTRACT

In one embodiment, a memory device includes a memory core and input receivers to receive commands and data. The memory device also includes a register to store a value that indicates whether a subset of the input receivers are powered down in response to a control signal. A memory controller transmits commands and data to the memory device. The memory controller also transmits the value to indicate whether a subset of the input receivers of the memory device are powered down in response to the control signal. In addition, in response to a self-fresh command, the memory device defers entry into a self-refresh operation until receipt of the control signal that is received after receiving the self-refresh command.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/276,338 filed on Feb. 14, 2019 which is a continuation of U.S. patentapplication Ser. No. 16/139,636 filed on Sep. 24, 2018 which is acontinuation of U.S. patent application Ser. No. 15/855,535 filed onDec. 27, 2017, which is a continuation of U.S. patent application Ser.No. 15/332,785 filed on Oct. 24, 2016, which is a continuation of U.S.patent application Ser. No. 14/573,323 filed on Dec. 17, 2014, which isa continuation of U.S. patent application Ser. No. 13/980,826 filed onJul. 19, 2013, which is a 35 U.S.C. 371 Patent Application of PCTApplication No. PCT/US2012/025310 filed on Feb. 15, 2012, which claimsthe benefit of U.S. Provisional Patent Application No. 61/445,947 filedon Feb. 23, 2011, each of which is incorporated by reference in itsentirety.

TECHNICAL FIELD

The present disclosure general relates to apparatus, protocols, andtechniques for providing power management in systems that include(integrated circuit) memory controller and memory devices.

BACKGROUND

As mobile devices continue to offer increasing processing power and datatransfer rates, battery life has become an important performance metric.Meanwhile, manufacturers strive to make mobile devices thinner andsmaller. Since the power reserve available in many mobile devices islimited by the energy density and size of its battery, power-managementfeatures of the underlying hardware can be useful improvements to systemblocks in order to increase the overall power efficiency of the mobiledevice.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates components in an exemplary memory system thatfacilitates separate control of power modes for a high-speed interface(HSI) of the memory component and operational states of the memory core,in accordance with one embodiment.

FIG. 2 presents an exemplary timing diagram illustrating the separatecontrol of power modes for the HSI and operational states of the memorycore, in accordance with one embodiment.

FIG. 3 presents an exemplary timing diagram illustrating the process ofcalibrating the receivers on the HSI without requiring the memory coreto exit its self-refresh mode, in accordance with one embodiment.

FIG. 4 presents an exemplary timing diagram illustrating how the clockfrequency can be controlled by a command carried on the CA bus upon theHSI exiting a power-down mode, in accordance with one embodiment.

FIG. 5 presents an exemplary timing diagram illustrating how to avoidvoltage-ramping interference upon the HSI exiting a power-down mode, inaccordance with one embodiment.

FIG. 6 presents an exemplary timing diagram illustrating the operationof a sideband bus when the HSI is in a low-power mode and the memorycore is in a low-power-consumption operational state, in accordance withone embodiment.

DETAILED DESCRIPTION

Embodiments of the present disclosure provide for various protocols andapparatus for memory device power management. In an embodiment, a memorysystem, memory devices, and controllers that control such memory devicesin the system allow a portion of a high-speed interface (HSI) of thememory device to be powered down. For example, a register is used tostore a value indicating the portion of the HSI to be powered down, anda power-mode signal is used to power down the corresponding HSI portion.The operational state of the memory core is controlled separately by acommand carried on the command/address (CA) portion and/or the dataportion of the HSI. This way, the HSI can be powered up from apower-down mode for calibration while the memory core remains in anoperational state with low power consumption.

FIG. 1 illustrates components in an exemplary memory system thatfacilitates separate control of power modes for a high-speed interfaceof the memory component and operational states of the memory core, inaccordance with one embodiment. In this example, a memory system 100includes a memory controller 102 and a memory device 106. Memory device106 includes a memory core 120, an interface circuit 114, and a numberof registers 128. Memory controller 102 includes an interface circuit115 and a control logic 104, which controls the power modes of interfacecircuit 114 and the operational state of memory device 106, as describedbelow. In one embodiment, memory controller 102 can communicate withmemory device 106 via interface circuits 115 and 114. Interface circuits115 and 114 are coupled to a clock (CK) signal line 112, a CA bus 110,and a DQ bus 108. During normal operation, the part of interface circuit114 which is coupled to CK signal line 112, CA bus 110, and DQ bus 108can operate at a high data rate and, in an embodiment, accommodate verylow-swing differential (VLSD) signals. In the following description,this part of interface circuit 114 is referred to as the high-speedinterface (HSI). Also coupled to memory device 106 is a sideband bus130, which includes an interface-power-mode (PM) signal line 126, asideband clock (SCK) signal line 124, and a sideband data (SDQ) bus 122.

Although in FIG. 1 interface circuit 114 is shown as a single block, inreality each signal line can couple to an interface on memory device106. In other words, each signal line can be considered as being coupledto a separate interface. Furthermore, CA bus 110 can be considered asbeing coupled to a unidirectional command/address interface (since CAbus 110 is typically used to transmit bits from memory controller 102 tomemory device 106). FIG. 1 illustrates CA bus 110 with a single arrow;however, addresses and commands can be conveyed over separate signallines or be multiplexed over the same or subsets of signal lines. DQ bus108 can be considered as being coupled to a bi-directional interface fordata transmission in both directions (which can also be two separateunidirectional read/write data interfaces). These interfaces may havesame or different data rates. In one embodiment, during a specificpower-down mode, a subset of the HSI transmitters and receivers can bepowered down while the rest can remain powered up. Alternatively, all ofthe HSI transmitters and receivers can be powered down. Memory device106 can provide this option to selectively power down portion(s) ofinterface circuit 114 as specified by values stored in a mode register,which can be one of the registers 128. In a further embodiment, thedifferent states of powering-down can be encoded in a power-downcommand. For example, a default command can be “power down all” (PDNALL)which powers down all portions of the HSI coupled to both CA bus 110 andDQ bus 108, and a “power down DQ” (PDNDQ) command can specify onlypowering down the HSI portion coupled to DQ bus 108. Note that thetransmitters and receivers associated with sideband bus 130 can remainpowered up when the HSI is powered down.

During operation, the level of PM signal 126, combined with the value ofthe mode register, determines the power mode for the HST. In general,the HST has at least two power modes: a power-up mode and a power-downmode. The power-up mode can correspond to several active states of thememory devices, such as idle (wherein the device is precharged), active(wherein a row has been activated), and active refresh (wherein a singlerow is being refreshed). In the power-up mode, all the transmitters andreceivers coupled to the HSI on both controller 102 and memory device106 are powered up. That is, the transmitters and receivers associatedwith DQ bus 108, CA bus 110, and CK bus 112 are all powered up. In thepower-down mode, the transmitters and receivers associated withdifferent buses can be selectively powered down. For example, when themode register is set to a certain value, only the transmitters andreceivers associated with DQ bus 108 are powered down in response to atransition in the level of the PM signal 126, whereas the transmittersand receivers associated with CA bus 110 remain powered up.Alternatively, when the mode register is set to a different value, allthe transmitters and receivers associated with the HSI, as well as thecorresponding transmitters and receivers in controller 102, are powereddown in response to a transition in the level of the PM signal 126.

Various methods can be used to power down a transmitter or receiver. Forexample, a receiver or transmitter can have a current source which canbe enabled or disabled (i.e., turned off) based on the power mode. Inaddition, the transmitter/receiver power for each individual interface(i.e., the interface corresponding to a given signal line or bus) can becontrolled separately. The internal clocking to a particular interfacecan be gated such that no clocking of the circuits in that interfaceoccurs when the interface is powered down.

In one embodiment, a high level on PM signal 126 indicates a normaloperation (power-up) mode, and a low level indicates a power-down modefor the HSI. A transitional edge of PM signal 126 triggers memory device106 to power down all or a subset of the transmitters and receiversassociated with the HSI, depending on the value stored in the moderegister. Correspondingly, memory controller 102 also turns off theassociated transmitters and receivers. Alternatively, memory controller102 can keep its transmitters and receiver powered on if the high-speedsignal lines are coupled to more than one memory device, so that memorycontroller 102 can communicate with other memory devices when one memorydevice has its HSI powered down. Because the transition of PM signal 126can occur very quickly, the HSI can be placed in the power-down modewith very little latency.

PM signal 126 does not affect the operational state of memory core 120.The operational state of memory core 120 is controlled by a commandcarried on CA bus 110 and/or DQ bus 108. For example, a self-refreshcommand can be transmitted by controller 102 on CA bus 110 to placememory core 106 in a self-refresh mode, before the HSI is put into thepower-down mode. Other commands can be used to place memory core 106 invarious states, such as idle standby and active standby. Such commandscan be stored in registers and be used at a later time to set or controlthe operational state of memory core 120. This configuration facilitatesseparate control of the power modes for the HSI and operational statesfor memory core 106. As a result, memory core 120 and the HSI can beturned “on” or “off” without affecting each other's power state. Whenthe HSI is in the power-down mode, memory core 120 is typically placedin an operational state with low power consumption. On the other hand,in certain situations, for example when the HSI needs to be calibrated,the HSI can be placed in a power-up mode (which can be triggered by arising edge of PM signal 126), while memory core 120 remains in thelow-power-consumption operational state. When memory core 120 is to exitthe low-power-consumption operational state, the HSI is typicallypowered up first, and a command is then transmitted via the HSI to wakeup memory core 120.

In one embodiment, memory controller 102 calibrates a set of parametersassociated with the transmitters and receivers of the HSI to optimizedata transmission. The calibration operations can be performed on aperiodic basis to accommodate changes in conditions such as voltage andtemperature fluctuation. To calibrate the HSI, controller 102 cantransmit test patterns on one or more signal lines coupled to the memorydevice via the HSI and receive results of sampled test pattern from thememory device over the HSI.

Parameters of the HSI may be adjusted and/or updated during thecalibration process and stored in registers. The calibration parameterscan include timing parameters, such as receiver sample phase andtransmitter drive phase, voltage parameters, such as receiver offset orreference voltage, receiver current bias, receiver terminationimpedance, transmit supply voltage, transmit drive swing voltage, andtransmit termination impedance.

The receiver sample phase is a parameter that affects the temporalposition of a received signal relative to a timing reference.Transmitter drive phase is a parameter that affects the temporalposition of a transmitted signal relative to a timing reference.Receiver offset is a parameter that adjusts the voltage level of areceived signal. Receiver reference voltage is an offset that adjusts areceiver reference voltage. Receiver current bias is a parameter thatadjusts the bias voltage and a current source for a receiver circuit.Receiver termination impedance is a parameter that affects the impedanceof a transmission line termination for a receiver circuit. Transmitsupply voltage is a parameter that affects the supply voltage for adriver used to transmit a signal. Transmit drive swing voltage is aparameter that affects the voltage swing of a transmitted signal by atransmitter. Transmit termination impedance is a parameter that affectsthe impedance of a transmission line termination on the transmitter (ordriver) circuit used to transmit a signal or the impedance of thetransmitter.

In some embodiments, SCK line 124 and SDQ bus 122 can remain functionalwhen the HSI is in the low-power mode and/or when memory core 120 is inthe low-power-consumption operational state. SCK signal 124 is typicallyat a frequency much lower than that of CK 112. Hence, sideband bus 130can remain operational at all times without being calibrated. Inaddition, SDQ bus 122 can be used to transfer data to and from registers128, even when memory core 120 is in a low-power-consumption operationalstate. This feature provides an alternative way to access registers 128without using the HSI.

FIG. 2 presents an exemplary timing diagram illustrating the separatecontrol of power mode for the high-speed interface and operational statefor the memory core, in accordance with one embodiment. In this example,the PM signal exhibits two transitions: a power-down entry 202 and apower-down exit 204. The memory controller changes the PM signal from ahigh level to a low level to place the HSI in a power-down mode.Transmission on the DQ bus, CA bus, and CK signal line is typicallyterminated before the PM signal transitions to the low level. Thispractice ensures that active bus transmissions are complete before thePM signal changes. As illustrated in FIG. 2, the transmission on the DQbus and transmission of valid commands on the CA bus are completed priorto the termination of the CK signal (by tcKsp as illustrated in FIG. 2).The falling edge of the PM signal can occur at tCKPM after thetermination of the CK signal.

After power-down entry 202, the HSI remains in a power-down state for aduration of tpD. However, the transition of the PM signal does notaffect the power state of the memory core. In general, the memory corecan be placed in a low-power self-refresh state by a command carried onthe CA bus when valid commands are allowed before the HSI enters thepower-down mode. For example, as illustrated in FIG. 2, a self-refreshentry (SRE) command can be placed on the CA bus to place the memory corein a low-power self-refresh mode. The memory core can also stay in anormal operational state while the HSI is in the power-down mode.

A rising edge of the PM signal triggers the power-down exit 204. AfterHSI exits the power-down mode, transmission on the HSI is resumed.Typically, to reduce the interference of voltage fluctuation and tominimize transmission errors, transmission on the HSI is slightlydelayed following the rising edge of the PM signal. In this example,transmission of the CK signal is resumed at tPMCK after power-down exit204. Transmission of valid commands on the CA bus is allowed at tpDxafter power-down exit 204. A self-fresh exit (SRX) command placed on theCA bus can bring the memory core out of the low-power self-refresh modeto resume normal operation.

One advantage of having separate control of the power modes for the HSIand operational states for the memory core is that it allows the HSI tobe periodically woken up for receiver calibration (such that the HSIremains locked with the clock in the memory controller) without wakingup the core. This feature saves both power and time. In conventionalsystems where the power modes of the HSI and operational states of thememory core are jointly controlled, each time the HSI receivers needcalibration, the memory core has to exit the low-power-consumptionstate. It could take the memory core hundreds of nano seconds to exitthe low-power-consumption state, while it only takes the HST tens ofnano seconds to exit the power-down mode for calibration. Hence,periodic calibration of HSI receivers in conventional systems can beboth energy-inefficient and time-consuming.

The present system solves this problem, because the PM signal can wakeup the HSI without waking up the memory core. FIG. 3 presents anexemplary timing diagram illustrating the process of calibratingreceivers on the high-speed interface without requiring the memory coreto exit its self-refresh mode, in accordance with one embodiment.Starting from the left side of FIG. 3, assume that the memory core is ina self-refresh mode and the HSI is in a power-down mode (period 302).Subsequently, when the receivers on HSI need to be calibrated in period304, the PM signal transitions to a high level, allowing the HSI to exitthe power-down mode. The memory controller then begins transmitting theCK signal and calibration bit sequence on both CA and DQ buses. Duringperiod 304, the memory core remains in the low-power self-refresh state.

After the HSI calibration is complete, the PM signal transitions to alow level so that the HSI can be placed back in the power-down modeduring period 306. The memory core also remains in the self-refreshstate. At the beginning of period 308, the PM signal transitions to thehigh level to power up the HSI. After the HSI is stabilized andfunctional, the memory controller transmits an SRX command via the CAbus to instruct the memory core to exit the self-refresh state andreturn to normal operation.

Although the example in FIG. 3 illustrates only one calibrationoperation (period 304), the memory system can perform recurring HSIcalibration in a similar way for an extended period. The energy and timesavings resulting from not having to wake up the memory core can besignificant.

Since the CA bus can carry various commands to control the power stateof the memory core, it is possible to change the operational state(e.g., operating frequency) of the HSI upon it exiting the power-downmode. FIG. 4 presents an exemplary timing diagram illustrating how theclock frequency can be controlled by a command carried on the CA busupon the high-speed interface exiting a power-down mode, in accordancewith one embodiment. In this example, prior to the HSI entering apower-down period 402, the memory controller issues aclock-modify-frequency (CKMF) command on the CA bus. This CKMF commandsets all the necessary parameters corresponding to the subsequentfrequency. Such parameters can include operating voltages, Row Addressto Column Address Delay (tRCD, measured in clock cycles), and accesstime for read data (tAC, measured in clock cycles). This information canbe stored in the registers within the memory core.

After the HSI power-down period 402, the memory controller changes thePM signal to a high level to bring the HSI back to the normal powermode. Correspondingly, the CK signal is transmitted at a differentfrequency. Before the CA bus and DQ bus can be used to transmit bits atthe new frequency, a calibration bit sequence is placed on these busesso that their receivers can be calibrated based on the new clock signal.

In the example in FIG. 4 the PM signal can be used as a way to triggerfrequency change on the HSI. That is, the PM signal can be used totemporarily “turn off” the HSI in preparation for a frequency change.During the HSI power-down period 402, the memory core may remain in aregular-power-consumption state. In some embodiments, the memory corecan also be placed in a low-power-consumption state when the HSI ispowered down. In such cases, the memory controller can issue an SREcommand after the CKMF command to place the memory core in thelow-power-consumption state, and an SRX command after the calibrationbit sequence to bring the memory core back to theregular-power-consumption state.

During the initial power-up of the memory device, the voltage ramp-upcan exhibit non-uniformities, as illustrated in the upper right cornerof FIG. 5. This voltage ramping period could take micro seconds. Toavoid interferences from this transition, the memory controller can rampup the voltage for its transmitter and receiver before setting the PMsignal to a high level. This way, the voltage is ramped up when thereceivers on the memory-core side are still in the power-down mode, andwill have stabilized when the HSI is powered on by a rising edge of thePM signal.

As illustrated in the example in FIG. 5, the memory controller can rampup the voltage for the HSI during a voltage ramping period 502. At thesame time, the PM signal remains at a low level. After the voltage hasstabilized, the memory controller changes the PM signal to a high levelto produce a power-on transition edge 504, which turns on all thereceivers for the HSI. Subsequently, transmission on the CK line, CAbus, and DQ bus can be resumed.

In some embodiments, the sideband bus can be used to transfer data toand from the memory core regardless of the power state of the HSI andmemory core. Referring back to FIG. 1, the SCK signal line 124 can carrya clock signal at a lower frequency than that of the HSI, henceobviating the need for the sideband bus to be calibrated. SDQ bus 122can be used to transfer data to and from registers 128. SCK signal line124 and SDQ 122 can remain operational even when the HSI is in thepower-down mode.

FIG. 6 presents an exemplary timing diagram illustrating the operationof a sideband bus when the high-speed interface is in a low-power modeand the memory core is in a low-power-consumption state, in accordancewith one embodiment. In this example, the sideband bus is used to carrycommands to change the HSI's operating frequency upon the HSI exitingthe power-down mode. As shown in FIG. 6, the HSI is powered down duringperiod 602. Prior to the HSI entering the power-down mode, the memorycontroller issues an SRE command on the CA bus to place the memory corein a low-power self-refresh mode. Meanwhile, the sideband clock (SCK)line carries a slow clock signal 604 to facilitate data transfer on thesideband data (SDQ) bus. The transferred data 603 includes one or morecommands and/or parameters necessary to change the HSI clock frequencyto a lower value. These commands and/or parameters can be stored in theregisters within the memory core.

Subsequently, the memory controller changes the PM signal to a higherlevel, which places the HSI in the power-up mode. The memory controllernow transmits the CK signal at a lower frequency. Since the memory corehas already received all the necessary parameters corresponding to thenew CK frequency, the memory core can now operate at this new frequency.The memory controller can then transmit an SRX command on the CA bus tobring the memory core out of the low-power self-refresh mode.

In further embodiments, the memory controller can read values from theregisters in the memory core via the sideband bus when the memory coreis in the low-power self-refresh mode. For example, some registers canstore information on the physical state, e.g., temperature, of thememory core. The sideband bus can be used to read the values of theseregisters, which allows the memory controller to monitor and maintainproper functionality in the memory core.

The above described embodiments may include fewer or additionalcomponents. Components may be combined into a single encapsulatedpackage, stacked on top of one-another in the same or different packetsand/or the position of one or more components may be changed. Ingeneral, a memory controller is a chip that orchestrates the control ofdata access to and from a memory device, which is an integrated circuitdevice having an array of memory cells. In some embodiments, the memorycontroller functionality is included in a processor or other integratedcircuit device, for example, a graphics processing unit (GPU), or amobile applications processor. Thus, there may or may not be astandalone memory controller in the memory system.

An output of a process for designing an integrated circuit, or a portionof an integrated circuit, comprising one or more of the circuitsdescribed herein may be a computer-readable medium such as, for example,a magnetic tape or an optical or magnetic disk. The computer-readablemedium may be encoded with data structures or other informationdescribing circuitry that may be physically instantiated as anintegrated circuit or portion of an integrated circuit. Although variousformats may be used for such encoding, these data structures arecommonly written in Caltech Intermediate Format (CIF), Calma GDS IIStream Format (GDSII) or Electronic Design Interchange Format (EDIF).Those of skill in the art of integrated circuit design can develop suchdata structures from schematic diagrams of the type detailed above andthe corresponding descriptions and encode the data structures on acomputer-readable medium. Those of skill in the art of integratedcircuit fabrication can use such encoded data to fabricate integratedcircuits comprising one or more of the circuits described herein.

While the present disclosure has been described in connection withspecific embodiments, the claims are not limited to what is shown. Forexample, in some embodiments the links between a memory controller and amemory device utilize half-duplex and/or full-duplex communication(e.g., communication on a given link may be in both directions).Similarly, the links between a memory controller and a memory device mayoperate at a data rate that is: a multiple of the clock frequency suchas double data rate (DDR), quad-data rate (QDR), or high multiple datarates.

Moreover, some components are shown directly connected to one another,while others are shown connected via intermediate components. In eachinstance the method of interconnection, or “coupling,” establishes somedesired electrical communication between two or more circuit nodes, orterminals. Such coupling may often be accomplished using a number ofcircuit configurations, as will be understood by those of skill in theart. Therefore, the spirit and scope of the appended claims should notbe limited to the foregoing description. Only those claims specificallyreciting “means for” or “step for” should be construed in the mannerrequired under the sixth paragraph of 35 U.S.C. § 112.

1. A dynamic random access memory device (DRAM) comprising: an array ofDRAM cells; a data interface to transmit, to a memory controller, dataaccessed from the array; a command interface to receive from the memorycontroller, a command that specifies a self-refresh operation of thearray; and a register to store a value that specifies whether to powerdown the data interface during the self-refresh operation.
 2. The DRAMof claim 1, wherein the DRAM defers the self-refresh operation untilreceipt of a control signal from the memory controller, the controlsignal received after receiving the command that specifies theself-refresh operation.
 3. The DRAM of claim 2, wherein the controlsignal specifies entry into a power mode in which the data interface orcommand interface is powered down.
 4. The DRAM of claim 3, wherein acombination of the value and a level of the control signal determinewhich of the data interface or the command interface is powered down inresponse to a first transition in the level of the control signal. 5.The DRAM of claim 4, wherein the value sets (i) that the commandinterface is to remain powered up, and (ii) that the data interface isto be powered down, in response to the first transition in the level ofthe control signal.
 6. The DRAM of claim 5, wherein the data interfaceis powered back up in response to a second transition in the level ofthe control signal while the DRAM performs the self-refresh operation,and the data interface receives a calibration value from the memorycontroller to calibrate at least one of the command interface or thedata interface without the DRAM exiting the self-refresh operation. 7.The DRAM of claim 2, further comprising: a sideband interface to receivethe control signal from the memory controller, wherein the sidebandinterface operates at a clock frequency that is less than a clockfrequency of at least one of the command interface and the datainterface.
 8. A method of operation in a dynamic random-access memorydevice (DRAM) having an array of DRAM cells, a data interface to outputdata accessed from the array, and a command interface to receivecommands, the method comprising: receiving from a memory controller, avalue that specifies whether to power down the data interface during aself-refresh operation of the array; and receiving from the memorycontroller, a command that specifies the self-refresh operation of thearray.
 9. The method of claim 8, further comprising: deferring theself-refresh operation until receipt of a control signal from the memorycontroller, the control signal received after receiving the command thatspecifies the self-refresh operation.
 10. The method of claim 9, whereinthe control signal specifies entry into a power mode in which the datainterface is powered down.
 11. The method of claim 10, furthercomprising: determining which of the data interface or the commandinterface is to be powered down in response to a first transition in alevel of the control signal based on a combination of the value and thelevel of the control signal.
 12. The method of claim 11, wherein thevalue sets (i) that the command interface is to remain powered up, and(ii) that the data interface is to be powered down, in response to thefirst transition in the level of the control signal.
 13. The method ofclaim 12, further comprising: powering back up the data interface inresponse to a second transition in the level of the control signal whilethe DRAM performs the self-refresh operation; receiving, at the datainterface, a calibration value from the memory controller to calibratethe command interface and the data interface without the DRAM exitingthe self-refresh operation.
 14. The method of claim 9, wherein thecontrol signal is received via a sideband interface that operates at aclock frequency that is less than a clock frequency of at least one ofthe command interface or the data interface.
 15. A dynamic random-accessmemory device (DRAM) comprising: a memory means for storing data; afirst interface means for transmitting, to a memory controller, the dataaccessed from the memory means; a second interface means for receivingfrom the memory controller, a command that specifies a self-refreshoperation of the memory means; and a means for storing a value thatspecifies whether to power down the first interface means during theself-refresh operation.
 16. The DRAM of claim 15, wherein the DRAMdefers the self-refresh operation until receipt of a control signal fromthe memory controller, the control signal received after receiving thecommand that specifies the self-refresh operation.
 17. The DRAM of claim16, wherein the control signal specifies entry into a power mode inwhich at least one of the first interface means or the second interfacemeans are powered down.
 18. The DRAM of claim 17, wherein a combinationof the value and a level of the control signal determine which of thefirst interface means or the second interface means are powered down inresponse to a first transition in the level of the control signal. 19.The DRAM of claim 18, wherein the value sets (i) that the secondinterface means is to remain powered up, and (ii) that the firstinterface means is to be powered down, in response to the firsttransition in the level of the control signal.
 20. The DRAM of claim 19,wherein the first interface means is powered up in response to a secondtransition in the level of the control signal while the DRAM performsthe self-refresh operation, and the first interface means receives acalibration value from the memory controller to calibrate at least oneof the first interface means or the second interface means without theDRAM exiting the self-refresh operation.